This invention relates to systems and techniques involving signal processing in single or multibeam sensing systems, and, more particularly, to systems and techniques involving signal processing of receive signals in single or multibeam sonar, radar and lidar (laser based radar systems) systems.
Briefly, by way of background, a sonar system may be used to detect, navigate, track, classify and locate objects in water using sound waves. Defense and civilian applications of sonar systems are numerous. In military applications, underwater sound is used for depth sounding; navigation; ship and submarine detection, ranging, and tracking (passively and actively); underwater communications; mine detection; and or guidance and control of torpedoes and other weapons. Most systems are monostatic, but bistatic systems may also be employed.
Civilian applications of underwater sound detection systems are numerous as well. These applications are continuing to increase as attention is focused on the hydrosphere, the ocean bottom, and the sub-bottom. Civilian applications include depth sounding; bottom topographic mapping; object location; underwater beacons (pingets); wave-height measurement; doppler navigation; fish finding; sub-bottom profiling; underwater imaging for inspection purposes; buried-pipeline location; underwater telemetry and control; diver communications; ship handling and docking aid; anti-stranding alert for ships; current flow measurement; and vessel velocity measurement.
A typical active sonar system includes a transmitter (a transducer commonly referred to as a "source" or "projector") to generate the sound waves and a receiver (a transducer commonly referred to as a "hydrophone") to sense and measure the properties of the reflected energy ("echo") including, for example, amplitude and phase. In a typical multibeam sonar system, a first transducer array ("transmitter or projector array") is mounted along the keel of a ship and radiates sound. A second transducer array ("receiver or hydrophone array") is mounted perpendicular to the transmitter array. The receiver array receives the "echoes" of the transmitted sound pulse, i.e., returns of the sound waves generated by the transmitter array. A conventional sonar system and transmitter and receiver array configuration is disclosed in Lustig et al., U.S. Pat. No. 3,114,631.
In those instances where the .transmitter array is mounted along the keel of the ship, the transmitter array projects a fan-shaped sound beam which is narrow in the fore and aft direction but wide athwart ship. The signals received by the hydrophones in the receiver array are summed to form a receive beam which is narrow in the across track but wide in the along track direction. The intersection of the transmit and receive beams define the region in the sea floor from where the echo originated. By applying different time delays to the different hydrophones signals the receive beams can be steered in different directions and when a number of receive beams are formed simultaneously they together with the transmit beam define the multibeam sonar geometry.
When the transmitted sound from the transmitter array is of a single frequency, the time delays can be translated into phase delays for beamforming the hydrophone signals from the receiver array. For a given frequency, the narrow width of the receive beam is governed by the number of hydrophones comprising the receiver array (i.e., the physical length of the receiver array) and the direction to which the beam is steered. A common rule of thumb for determining the receive beam width (in degrees) is ##EQU1## where: (1) "a" is the length of the array;
(2) ".lambda." is the wavelength (determined by the frequency of the sound wave of the projector) in the same units as "a" (the length of the array); and PA1 (3) ".theta." is the direction of the beam steer. PA1 y.sub.n =the value of the extrapolated processing unit; PA1 y.sub.n-k =the value of the extreme points of the physical and/or previously extrapolated processing unit in the direction of the extrapolation; PA1 d.sub.k =the N.sup.th order predictor coefficients that predict the next value y.sub.n of the spatial series from the previous N values y.sub.n-k, k=1 to N of the extreme physical and/or extrapolated processing units; PA1 M=the number of physical sensors in the physical array contributing to the calculation of the prediction coefficients d.sub.k, k=1 to N, and N&lt;M; and PA1 y.sub.n =the value of the extrapolated processing unit; PA1 y.sub.n-k =the value of the extreme points of the physical and/or previously extrapolated processing unit in the direction of the extrapolation; PA1 d.sub.k =the N.sup.th order predictor coefficients that predict the next value y.sub.n of the spatial series from the previous N values y.sub.n-k, k=1 to N of the extreme physical and/or extrapolated processing units; PA1 M=the number of physical sensors in the physical array contributing to the calculation of the prediction coefficients d.sub.k, k=1 to N, and N&lt;M; and
Thus, it can be seen that for narrower beam widths the length of the receiver array should be larger. Stated simply, a "narrower" beam width of the receiver beam increases the information that may be obtained about the reflecting objects, e.g., object resolution, accuracy of object direction, and range coverage. However, in many applications of multibeam sonars, the physical characteristics of the receiver array are constrained by the physical characteristics of the ship. For example, in many instances where the receiver arrays are mounted athwart ship for multibeam sonars, the maximum physical length of the array is restricted by the width of the ship. The physical characteristics of the receiver array may also be restricted as a result of the draft of the ship being constrained. This tends to confine the depth of the receiver array and may require segmentation of the receiver array.
Further, in many instances maintaining the structural integrity of the keel of the ship may impact upon the physical characteristics of the receiver array. Maintaining the structural integrity of the keel is important for those ships employed as "ice-breakers". In this situation, the hydrophone array may not be installed athwart ship as a single unit. Instead, the receiver array may be divided into two or more sub-arrays each array conforming to the hull of the ship. However, installing the receiver array as two or more sub-arrays, without a continuous locus of data athwart ship for multibeam sonars, may cause an increase in the beam pattern distortions, e.g., an increase in the side lobes of the calculated receive beam.
There exists a need for a signal processing system and technique to reduce the effective beam width of the receive beam without increasing the physical dimensions of the array. Further, there exists a need for a signal processing system for a sonar system that overcomes many typical constraints imposed upon the structural characteristics of the receiver array.